Contents lists available at ScienceDirect Geothermics journal homepage: www.elsevier.com/locate/geothermics The injection of CO 2 to hypersaline geothermal brine: A case study for Tuzla region Gokhan Topcu a , Gonca A. Koç b , Alper Baba c , Mustafa M. Demir a, ⁎ a Department of Materials Science and Engineering, İzmir Institute of Technology, 35430, Gülbahçe, Urla, İzmir, Turkey b Tuzla Geothermal Power Plant, 35210, İzmir, Turkey c Department of Civil Engineering, İzmir Institute of Technology, 35430, Gülbahçe, Urla, İzmir, Turkey ARTICLE INFO Keywords: CO 2 Capture CO 2 Geothermal energy Geothermal scaling High salinity brine ABSTRACT Scaling is a serious issue for geothermal power plants since it remarkably decreases the harvesting of energy. The reduction of pH by organic acids whose structure is close to CO 2 for instance formic acid has been an effective solution for the minimization of scaling. Herein, the effect of CO 2 injection on the formation of scaling parti- cularly metal-silicates was investigated for the model case of Tuzla Geothermal Field (TGF) located in the northwest of Turkey. CO 2 has an acidic character in aqueous systems because it leads to the formation of car- bonic acid. The injection of 20.6 m 3 /s CO 2 (approximately 88 ppm) to hypersaline brine of TGF is a promising green approach for both mitigation of scaling by reducing pH from 7.2 to 6.2 at the well-head and the mini- mization of potential corrosion compared to the use of formic acid (55 ppm). 1. Introduction Geothermal energy is a sustainable resource by which electricity can be generated from the heat stored by water reservoirs (Lund and Boyd, 2016). The most important operational handicap of power plants is the scaling by components such as calcite, silicate, metal-silicates, and sulfides (Gallup, 2002; Potapov et al., 2001). CO 2 that is an acidic gas is separated from the brine due to the decrease in pressure and tem- perature (Baba et al., 2015; Duan and Sun, 2003). This elimination leads to an increase in the pH of the brine, hence decreasing the solu- bility of minerals, particularly silica, in the water. As the geothermal fluid is transported to the power plant through the well, minerals with low solubility begin to precipitate in an uncontrollable manner on the equipment, called scaling (Demir et al., 2014). This phenomenon causes a blockage in the pipes of the heat exchangers, where the energy transfer of the system takes place. Furthermore, this deposit formation reduces the inner width of the system and prevents heat transfer, thereby reducing the efficiency of electricity production. For the cleaning of metal silicate scaling, the production system is closed for a while and the system is washed with acids to dissolve silica and eliminate the metal-silicate deposit, which has high mechanical resistivity and stable chemical nature (Demadis et al., 2011a). For in- stance, hydrofluoric acid (HF) disintegrates the Si-O bonds are replaced by Si-F bonds. (Knotter, 2000) Nevertheless, the acid treatment in- creases the operating cost of the system; moreover, this operation leads to the loss of production during a period of stoppage. In addition, the installation of the plant, which is costly and laborious, is damaged by this process because these chemicals increase the risk of corrosion in pipes (Demadis et al., 2011b; Zhang et al., 2011). In this wise, a number of the study was performed in the literature to mitigate the metal-sili- cate scaling using various strategies involving the use of inhibitors and regulation of water (Demadis et al., 2012; Gallup and Barcelon, 2005; Topçu et al., 2017). For the former method, organic molecules are commonly employed to eliminate the scaling using either chelating or dispersion mechanisms. In general, these compounds include proto- nated primary, secondary or tertiary amines (Danilovtseva et al., 2011; Demadis et al., 2008; Spinde et al., 2011), amide moieties (Demadis and Neofotistou, 2007), phosphonate (Spinthaki et al., 2018), ether (Preari et al., 2014), or alcohol functional groups (Topcu et al., 2019). For the latter, the brine is regulated using weak organic acids to reduce pH and increase the solubility. For instance, the acidification of high salinity brine to mitigate silicate scaling has been examined in our previous study. The scaling was remarkably prevented by reducing the pH of brine to < 6 using 55 ppm formic acid (HCOOH) (Baba et al., 2015). Formic acid is the simplest organic acid, whose structure is close to CO 2 . Moreover, it is a ferric iron reducing agent, and ferric silica scale may be more insoluble than ferrous silicate scale. The acidity of the additive is quite an important parameter for preventing the formation of the geothermal deposit. Carbonic acid (H 2 CO 3 ) has moderate acidity and simple chemical structure, compared https://doi.org/10.1016/j.geothermics.2019.02.011 Received 16 December 2018; Received in revised form 28 January 2019; Accepted 20 February 2019 ⁎ Corresponding author. E-mail address: mdemir@iyte.edu.tr (M.M. Demir). Geothermics 80 (2019) 86–91 Available online 28 February 2019 0375-6505/ © 2019 Elsevier Ltd. All rights reserved. T